1. Field of the Invention
The present invention generally relates to duplexers using a band-pass type of surface acoustic wave filter (SAW filter), and more particularly, to a duplexer with an impedance matching circuit.
2. Description of the Related Art
In recent years, cellular phones and portable information terminals have spread with the development of mobile communication systems, and manufacturers compete with one another for downsizing and higher performance of these terminals. Some cellular phones operate in both the analog and digital systems and use both a band of 800 MHz to 1 GHz and another band of 1.5 GHz to 2.0 GHz.
Now, there is considerable activity in the development of dual-mode or dual-band cellular phones for improvement in performance. The dual-mode cellular phone operates in the analog and digital systems, or digital TDMA (Time Division Multiplexing Access) and CDMA (Code Division Multiplexing Access). The dual-band cellular phone operates in, for example, the 800 MHz and 1.9 GHz bands, or 900 MHz and 1.8 or 1.5 GHz bands. These cellular phones require higher-function components, which are simultaneously required to be downsized and cost-reduced.
In cellular phones as described above, a duplexer is used to branch off a signal in two ways. Generally, the duplexer is equipped with a filter and a phase (impedance) matching circuit. Many filters have a band-pass filter, a band-rejection filter or the combination thereof, any of which uses a dielectric substance. However, the recent years have seen a remarkable progress in development of SAW filters, which enable downsizing and higher performance.
FIGS. 1A and 1B show a conventional duplexer. More particularly, FIG. 1A shows a block diagram of the structure of the duplexer, and FIG. 1B is a graph of the frequency characteristic thereof. In FIG. 1B, the horizontal axis denotes frequency which becomes higher rightwards, and the vertical axis denotes the pass intensity which becomes stronger upwards. As shown in FIG. 1A, a duplexer 10 is made up of two filters 12 and 13, an impedance matching circuit (hereinafter simply referred to as matching circuit) 11, a common terminal 14 and individual terminals 15 and 16. The filters 12 and 13 are formed by SAW filters, and have different center frequencies f1 and f2 of the respective pass bands, wherein f2 is higher than f1 (f2>f1).
The matching circuit 11 is used to prevent the filter characteristics of the filters 12 and 13 from being degraded. It is now assumed that Z1 is defined as the characteristic impedance defined by viewing the filter 12 from the common terminal 14, and Z2 is defined as the characteristic impedance defined by viewing the filter 13 therefrom. Due to the function of the matching circuit 11, the characteristic impedance Z1 of the filter 12 matches with the characteristic impedance at the common terminal 14, while the characteristic impedance of the filter 13 is indefinite and the reflection factor is equal to 1 when the signal applied to the common terminal 14 has the frequency f1. In contrast, for the signal of the frequency f2 applied to the common terminal 14, the characteristic impedance on the filter 12 is indefinite and the reflection factor is equal to 1, while the characteristic impedance of the filter 13 matches with that of the common terminal 14.
The duplexer shown in FIG. 1A is disclosed in, for example, Japanese Unexamined Patent Publication Nos. 6-310979 and 10-126213. The publications show a downsized structure with a multilayer ceramics package and an impedance matching circuit formed by a strip line or microstrip line. Further, the publications show a structure of the matching circuit line that is formed on an intermediate layer embedded in the multilayer structure and is interposed between the upper and lower ground patterns. More particularly, the upper and lower ground patterns are formed on the layers adjacent to the layer on which the matching circuit pattern is formed so that the line is interposed therebetween.
However, the above-mentioned structure has a disadvantage resulting from the structure in which the upper and lower ground patterns are close to the impedance matching circuit line. The close location causes reduction in the characteristic impedance of the matching circuit line and degrades the filter characteristic. Using thicker layers of the multilayer package in order to increase the distance separation between the matching circuit line and the ground patterns may eliminate this problem. However, the above solution makes thicker the multilayer package and makes difficult downsizing.